A form of PNEUMONIA due to the bacterium Legionella pneumophila, so-called because the first identified outbreak was in a group of US ex-servicemen (members of the American Legion). Inhalation of water aerosols seems the most likely way of acquiring the disease, for example from air conditioning outlets. In the presence of the disease, the treatment of infected water systems is essential by cleaning, chlorination, heating or a combination of all three.

The pneumonia has no distinctive clinical or radiological features, so the diagnosis is based on an antibody test performed on a blood sample. There is no evidence that the disease is transmitted directly from person to person. The incubation period is 2–10 days, the disease starting with aches and pains, followed rapidly by a rise in temperature, shivering attacks, cough and shortness of breath. The X-ray may show patchy areas of consolidation in the lungs. Erythromycin and rifampicin are generally prescribed.